fall behind vs drop off

fall behind and drop off both are verbs.

Word	Noun	Adjective	Verb	Adverb
fall behind	No	No	Yes	No
drop off	No	No	Yes	No

As verbs, fall behind and drop off are synonyms defined as:

Other synonyms of fall behind include fall back, recede.

fall behind (verb)	drop off (verb)
retreat	change from a waking to a sleeping state
hang (back) or fall (behind) in movement, progress, development, etc.	fall to a lower standard
	fall or diminish
	retreat
	remove (cargo, people, etc.) from and leave